Possibly related upstream bugs: Bug 331257 - data: images show up when 'load images' (Tools->Options->content->checkbox) is disabled Bug 449273 - SVG:display desc (and title?) as text if images are blocked Bug 1196784 - img tag with srcset attribute bypasses permissions.default.image=2
https://bugzilla.mozilla.org/buglist.cgi?bug_id=331257,449273,1196784
